1. Field of the Invention
This invention relates to the positioning a mobile fastener applicator with respect to a workpiece and is particularly, but not exclusively, concerned with automated fastener application and the robotic presentation of an applicator to a workpiece.
The term workpiece is used herein to embrace one or more workpiece elements to be fastened together.
2. Description of the Prior Art
Robotic and like devices are capable of bringing an applicator precisely into a predetermined position, but normally have no means of subsequently adjusting that position if the workpiece is not equally precisely positioned.
Hence, there is a requirement for a mechanism which, immediately prior to applying a fastener, affords the applicator freedom of ultimate positional adjustment, to accommodate workpiece positional variation.
Another requirement of automated application systems is a short fastener application operating cycle time capability.
In order to achieve this, both a fastener driving plunger and opposed die of an applicator must be moved clear of an applied fastener (and dear any material, eg metal, displaced during the fastening cycle) immediately the fastener has been applied--so that the applicator is free to move to the next fastening position without delay.
A further requirement of such automated application systems is a capability, without risk of workpiece damage, of bringing the applicator into contact with some datum point on a workpiece, for optimal fastener positioning.
An example of this requirement is shown in FIG. 3A, in which fasteners are to be applied to join the flange sections 35 of two sheet metal pressings 36.
If the precise position of the main body of the pressings 36 is uncertain, it would be dangerous for the robot to bring the applicator in to its final position under power.
Rather, there is a need for a subsidiary mechanism which can take over the positioning of the applicator when the robot has brought it to the nearest safe position and can then bring it gently into contact with the main body of the pressings 36.
In this way, the flange 35 width can be kept to a minimum and a fastener (not shown) can be positioned in its most effective position, immediately adjacent to the main body of the pressings 36.
Hitherto known automated or robotic positional devices may generally perform satisfactorily when workpieces are correctly positioned, or can be forced into the correct position without damage.
However, there are many workpieces, such as the pressed steel sections used in the construction of motor vehicles and domestic appliances, which cannot be manufactured economically with the positional accuracy necessary for optimum fastening.
In such cases, the use of rigid fastening devices can lead to workpiece damage--unless the workpiece is enlarged, to allow greater tolerance in the positioning of fastening points.
A conventional solution has been to mount the applicator on a spring system, which allows a certain amount of applicator movement relative to a robot mounting arm.
However this approach has serious limitations--in that any given spring system can hold an applicator in equilibrium only in any one orientation. Accordingly, if fasteners have to be applied in more than one plane, a spring-based system is inappropriate.
Furthermore, a typical applicator can weigh from 20-50 kgs, requiring heavy springing to support it--and consequently relatively high forces and torques to reposition it.
The workpieces have to be clamped between the fastener delivery nose and opposed setting die of the applicator, prior to the fastener being applied.
Both the nose and the die are normally small in diameter, in order to facilitate access to the fastening points, and hence can generate high positioning torques only by exerting on the workpiece loads which may cause surface damage.